Apparatus and method for applying feedback control to a magnetic lens

ABSTRACT

An apparatus configured to control a magnetic field strength of a magnetic lens is provided. The apparatus may include a magnetic sensor configured to generate an output signal responsive to a first magnetic field strength of the magnetic lens. The apparatus may also include a control circuit coupled to the magnetic sensor and the magnetic sensor. The control circuit may be configured to receive the output signal from the magnetic lens and to receive an input signal responsive to a predetermined magnetic field strength. The control circuit may be further configured to generate a control signal responsive to the output signal and the input signal. Additionally, the control circuit may be configured to apply a current to the magnetic lens such that a second magnetic field strength may be generated within the magnetic lens closer to the predetermined magnetic field strength than the first magnetic strength.

This application claims benefit of U.S. Provisional 60/212,104 filedJun. 15, 2000.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention generally relates to a magnetic lens which may beconfigured to apply a magnetic field to a charged particle beam, andmore particularly, to a sectored magnetic lens and a control apparatusfor a magnetic lens which may be incorporated into a scanning electronmicroscope system.

2. Description of the Related Art

As the dimensions of semiconductor devices continue to shrink withadvances in semiconductor materials and processes, the ability toexamine microscopic features and to detect microscopic defects hasbecome increasingly important in the successful fabrication of advancedsemiconductor devices. Significant research continues to focus onincreasing the resolution limit of metrology tools that are used toexamine microscopic features and defects. Optical microscopes generallyhave an inherent resolution limit of approximately 200 nm and havelimited usefulness in current manufacturing processes. Microscopes thatutilize electron beams to examine devices, however, may be used toinvestigate feature sizes as small as, e.g., a few nanometers.Therefore, tools that utilize electron beams to inspect semiconductordevices are increasingly becoming integral to semiconductor fabricationprocesses. For example, in recent years, scanning electron microscopyhas become increasingly popular for the inspection of semiconductordevices. Scanning electron microscopy generally involves scanning anelectron beam over a specimen and creating an image of the specimen bydetecting electrons that are reflected, scattered, and/or transmitted bythe specimen.

The electron optical system of a scanning electron microscope generallyincludes an electron source, a device or a plurality of devicesconfigured to focus an electron beam generated by an electron source, adetector or a plurality of detectors configured to detect electronsreflected, scattered, or transmitted by the specimen, and a controlsystem. A thermal field emission source may typically be used as anelectron source, and the energy of the electron source may be controlledby an emission control electrode and an anode. The electron beam maypass through a magnetic condenser lens configured to collimate theelectron beam. An initial deflection system may also be located near theelectron source. An initial deflection system may be configured tocorrect alignment, stigmation and blanking of the beam. Prior to passingthrough a magnetic objective lens, the beam may also be passed through abeam limiting aperture and one or more electrostatic pre-lensdeflectors. The magnetic objective lens may further focus the electronbeam to a spot size of, for example, approximately five nanometers. Asused herein, the term “spot size” is generally defined as a lateraldimension of an electron beam incident upon a specimen. A magneticobjective lens may typically include a lower pole piece, an intermediateelectrode, and an upper pole piece.

An electron beam exiting a magnetic objective lens may be scanned acrossa specimen. Typically, the electron beam may be scanned in a firstdirection while the stage supporting the specimen may be moved in adirection perpendicular to the first direction. A plurality of detectionsystems may be used to detect secondary electrons, back-scatteredelectrons, and transmitted electrons that may be produced when theelectrons contact the specimen. Examples of scanning electron microscopesystems are illustrated, for example, in U.S. Pat. No. 4,928,010 toSaito et al., U.S. Pat. No. 5,241,176 to Yonezawa, U.S. Pat.No.5,502,306 to Meisburger et al., U.S. Pat. No. 5,578,821 to Meisburgeret al., U.S. Pat. No. 5,665,968 Meisburger et al., U.S. Pat. No.5,717,204 to Meisburger et al., U.S. Pat. No. 5,869,833 to Richardson etal., U.S. Pat. No. 5,872,358 to Todokora et al., and U.S. Pat. No.5,973,323 to Adler et al., and are incorporated by reference as if fullyset forth herein.

The performance of a scanning electron microscope may vary depending on,for example, the capability to focus an electron beam on a small targetarea. High voltage electrons may penetrate deep into a semiconductorsubstrate or a portion of a semiconductor formed upon a semiconductorsubstrate thereby damaging the substrate or the device and rendering itunsuitable as a working device such as an integrated circuit. Therefore,low voltage electron beams may typically used to analyze delicatesemiconductor specimens that otherwise might be damaged by high voltageelectron sources. The primary factor that reduces resolution in the lowacceleration voltage region is blur of the electron beam due tochromatic aberration. Dispersion in the energy of the electron beamemitted from the electron source typically causes chromatic aberration.As such, significant effort has been focused on improving theperformance of a scanning electron microscope by enhancing the abilityof the magnetic objective lens to reduce chromatic aberrations in anelectron beam source especially in low voltage particle beams.

Traditionally, magnetic lenses may be axially symmetric and may produceaxially symmetric magnetic potentials and magnetic fields. An example ofsuch a magnetic lens is illustrated, for example, in U.S. Pat. No.6,002,135 to Veneklasen et al. and is incorporated by reference as iffully set forth herein. A magnetic lens may include an inner pole piecethat may have a cylindrical upper portion and a conical lower portionthat may be substantially enclosed by an outer pole piece. The outerpole piece may also have a cylindrical upper portion and a conical lowerportion corresponding to the inner pole pieces. A solenoidal excitationcoil may be disposed between the inner pole piece and the outer polepiece. When a current is applied to the excitation coil, an axialfocusing field may be generated within the lens by magnetic flux fromthe inner and outer pole pieces. The axial focusing field may be used tofocus an electron beam. Shielding rings may be arranged between theupper and lower portions of the inner pole piece to reduce the air gapbetween the pole portions. The shielding rings may also provide a returnpath for deflection flux that may otherwise radiate through the gap andinduce eddy currents in outer pole pieces and excitation coil.Deflection coils may also be included within the lens along the beampath.

Variable axis lenses have also been developed to focus electron beams.Variable axis lenses incorporate supplementary lenses or supplementarydeflectors in the magnetic lens to provide some correction of electronbeam paths that may be laterally displaced from an optical axis of thelens. The supplementary lenses and deflectors may be energized based onthe lateral displacement of the beam path. Although electron beams maybe deflected by this lens, astigmation may still be a problem.Therefore, a separate astigmation compensator may also be included insuch a lens. Alternatively, an astigmatism-correction deflector systemmay be arranged within a variable axis lens adjacent the internalsurface of the supplementary deflectors. Such deflectors may beconstructed of an octapole three-stage coil in which each octapoleincludes two tetrapole sets. A deflection field coil may be added to oneof the tetrapole coil sets of the octapole. An example of a variableaxis lens is illustrated in U.S. Pat. No. 5,952,667 to Shimizu and isincorporated by reference as if fully set forth herein. Theincorporation of a separate astigmator octapole forces the beam to passthrough the center of this octapole. The overall alignment of the lenssystem, however, may be non-colinear due to the incorporation of such aseparate feature. Therefore, complexity of the overall alignment of thesystem increases when the charged particle beam is forced to passthrough successive non-colinear points.

There are, however, several disadvantages to the lens systems describedabove. For example, axially symmetric lenses may typically suffer fromhysteresis, large inductance of the excitation coil, and thermalstability problems. Hysteresis may cause a relationship between theexcitation coil current and the deflected beam position to depend uponpast deflection history. Therefore, accurate focus of an electron beamusing the magnetic lens may be extremely difficult to maintain andcontrol. Additionally, large inductance of the excitation coil may causethermal stability problems due to heat generated by the lens or theexcitation coils. Therefore, the center of the lens may shift due tothermal expansion of the materials used to construct the lens.

Immersion lenses are also limited in their application to a variety ofspecimens. Immersion lenses are generally designed to limit aberrationsof an electron beam by reducing the distance between the specimen andthe maximum magnetic field. The distance between the specimen and themaximum magnetic field may be reduced by placing the specimen near orwithin the lens. Examples of immersion lenses are illustrated in U.S.Pat. No. 5,089,428 to Da Lin et al. and is incorporated by reference asif fully set forth herein. Due to space limitations, immersion lensesmay not be able to accommodate a large specimen such as a semiconductorsubstrate. For example, 200 mm wafers, or semiconductor substrates, arealready being used in the development and production of semiconductordevices. Efforts are also underway to further increase the size ofsemiconductor substrates to 300 mm. Modifying these lenses in order toaccommodate such large semiconductor substrates may also adverselyaffect the performance of immersion lenses. Alternatively, reducing thesize of the semiconductor substrate by cross-sectioning the wafer is notusually an option due to the cost associated with destroying a productwafer.

An asymmetric immersion lens may be configured to reduce the distancebetween a specimen and the strongest magnetic field of the lens. Anasymmetrical lens, however, may be configured to produce a magneticfield that rises sharply just in front of a conical pole piece near thebore of the lens or the position at which the electron beam exits themagnetic lens. The magnetic field falls slowly toward a second polepiece or a magnetic housing. The specimen and the conical pole piece aredisposed within the magnetic housing such that the specimen may beplaced near the conical pole piece. Asymmetric immersion lenses may bemore flexible to accommodate large specimen such as semiconductorsubstrates, but these lenses may have a reduced capability to detectsecondary electrons. For example, because secondary electrons may beemitted at a point beyond the magnetic field peak, low energy secondaryelectrons may not be able to surmount the magnetic field maximum. Inorder to overcome low detection of secondary electrons, a conductinggrid system may be included in the lens. The conducting grid system mayinclude an auxiliary grid to accelerate the secondary electrons awayfrom the inner surface of the lower pole piece, an extraction grid toreduce the axial velocity of the secondary electrons, and a restraingrid to turn back any uncollected secondary electrons. Therefore, inorder to overcome the low detection of secondary electrons, a conductinggrid system may be included in the lens.

In addition to the above disadvantages, the performance of magneticlenses may also be limited due to changes in the magnetic field strengthdue to low frequency noise, drift in the performance of current driveelectronics, drift due to eddy currents or superimposed fields fromother sources, and drift in the magnetic field strength over time fromother causes. Although a magnetic lens design may minimize these effectson the performance of the lens, it may not be possible to substantiallyeliminate magnetic field drift of the lens. For example, eddy currentsdue to magnetic flux leakage from a lens through a gap in the magneticlens may adversely affect the performance of the magnetic lens. Becausea magnetic lens must be designed with a bore to allow the electron beamto travel through the lens, however, it is impossible to seal themagnetic lens off completely. As a result, some of the magnetic fieldwill inherently “leak” out of a magnetic lens. Therefore, the effects ofeddy currents on the performance of a magnetic lens may not becompletely eliminated due to usage requirements. Drift in the magneticfield may cause the electron beam to drift out of focus. Therefore, theoverall resolution of a scanning electron microscope may also be reducedby the presence of the above sources of magnetic field drift. Thefunctioning of the scanning electron microscope may, however, bedramatically improved by an accurate control system for the magneticobjective lens.

A control mechanism for a magnetic lens may generally include a devicefor sensing the current density of the electron beam at a positionspaced from an axis along which an electron beam travels. For example,an alignment yoke disposed along the axis of the electron beam mayreceive a signal from the current sensing arrangement. Therefore, thealignment yoke may be mechanically shifted incrementally andorthogonally until a maximum current may be produced at the referencelocation. The beam may be centered due to the altered position of thealignment yoke thereby reducing aberrations in the projections lens. Anexample of such a focusing system is illustrated in U.S. Pat. No.4,423,305 to Pfeifer and is incorporated by reference as if fully setforth herein. Mechanical focus methods, however, may be slow due to thetime required for moving the devices. In addition, microscopicvibrations due to mechanical motion of the alignment yoke may need tosettle before the magnetic lens may perform adequately. Therefore,mechanical focus methods may require additional time such thatmicroscopic vibrations will not affect the performance of the magneticlens.

An alternative control mechanism for a magnetic lens involves focusingand controlling an electron beam by determining the focal length atwhich a sample will be brought into focus. Focal length may be afunction of the electron beam energy and the magnetic field strength. Inthis manner, one available control mechanism involves using an electrontrajectory tracing program to measure the converging point, or focallength, for an electron beam by using measurements of the electron beamenergy and the magnetic field strength. The magnetic field strength maybe estimated by measuring a current in the lens coil. An adjustment tothe current in the lens coil may be made to correct the converging pointof the electron beam. Instantaneous magnetic field strength, however,may be determined by the present current and the history of all othercurrents in the coil which is commonly referred to as hysteresis.Hysteresis in the magnetic field strength may also be induced fromfrequent changes in the voltage supplied to a magnetic lens. Forexample, the lens current in a scanning electron microscope may often beautomatically adjusted to a nominal beam voltage. As new specimens arebeing observed, the user may usually alter the beam voltage to bring thespecimen into focus. Therefore, frequently adjusting the beam voltagemay increase the complexity of a current history of the lens. As thecomplexity of the current history increases, hysteresis will also becomemore problematic. Therefore, estimating the magnetic field strengthusing measurements of the current in the lens coil may induce error inthis method. In addition, the coil and the core materials may react in anon-ideal way to the frequent changes in the current being supplied tothe coil. Therefore, additional electrical and magnetic characterizationof the coil and the core materials may be necessary. A thoroughdegaussing procedure may reduce the effects of hysteresis, however,magnetic hysteresis typically remains a problem in most magnetic lenses.

Additional methods to control the electron beam focus have attempted toreduce the effects of hysteresis of the magnetic lens by keeping thelens current constant after an initial manual focus and calibration. Anexample of such a magnetic lens control method is illustrated in U.S.Pat. No. 4,999,496 to Shaw et al. and is incorporated by reference as iffully set forth herein. The control method involves varying the electronbeam energy to alter the focus of the magnetic lens, as working distancechanges such as when different areas of a specimen are viewed. In orderto offset the effects of a new beam energy, the current in the scanningcoils may be altered to maintain accurate magnification. Although such amethod for focusing the electron beam may reduce deleterious effects ofhysteresis in the current of the magnetic objective lens, other factorsthat may lead to defocusing may not be addressed in this design. Forexample, as mentioned above, other factors that may hinder theperformance of a magnetic lens may include thermal changes in thematerial properties, drift in the current drive electronics, drift inthe magnetic field due to eddy currents, and drift due to superimposedfields from other sources.

Furthermore, in many scanning electron microscope systems, coarse andfine focusing of the magnetic lens may typically be performed manuallyby an operator. The operator may alter the focus of the magnetic lens bycontrolling the electric current of the magnetic lens. In order toobtain the desired magnetic field strength, the operator may alter thecurrent while observing the effects of the magnetic field on an image ofa specimen until the optimal performance is achieved. The image may beobserved using a display system such as a color or grayscale monitor.The resulting magnetic field, however, may follow a nonlinearrelationship with the current due to hysteresis in the magnetic lens andthe behavior of the coil and the core materials in response to thechange in current. This iterative method also depends on operatorjudgement and is consequently subject to error. More importantly, theadditional sources of magnetic field drift described above may alsocause this magnetic field to be irreproducible.

Accordingly, it would be advantageous to improve the performance ofmagnetic lens and a magnetic lens control apparatus that may be used tofocus an electron beam by reducing the effects of, for example,hysteresis and thermal instability in the magnetic field strength of amagnetic lens.

SUMMARY OF THE INVENTION

In an embodiment, a magnetic lens such as a magnetic circuit configuredto apply a magnetic field to a charged particle beam and an apparatusconfigured to control a magnetic field strength of a magnetic lens areprovided. The charged particle beam may be a beam of ions or a beam ofelectrons. The charged particle beam may travel through the magneticlens from a first end of the magnetic lens to a second end of themagnetic lens. The magnetic lens and the apparatus may be configured tobe incorporated into a scanning electron microscope.

In an embodiment, the magnetic lens may have an outer pole piece and aninner pole piece. The outer pole piece may be coupled to the inner polepiece. The outer pole piece may have at least two sectors and at leasttwo slots. In addition, the outer pole piece may have eight sectors andeight slots. Each sector may be disposed between lateral boundaries oftwo slots in the outer pole piece such that the magnetic potential ofeach sector may be substantially independent of the magnetic potentialof each other sector on the outer pole piece. Furthermore, the innerpole piece may also have at least two sectors and at least two slots. Inaddition, the inner pole piece of the magnetic lens may have eightsectors and eight slots.

In an embodiment, the magnetic lens may include a primary coil winding.The primary coil winding may be interposed between the outer pole pieceand the inner pole piece of the magnetic lens. The primary coil windingmay be configured to drive a magnetic potential of the outer pole piecerelative to the inner pole piece when a current is applied to theprimary coil winding. The magnetic lens may also include at least twosector coil windings. Each sector coil winding may be coupled to onesector of the outer pole piece. In addition, if the inner pole piecealso has sectors, then a sector coil winding may be coupled to eachsector of the inner pole piece. In this manner, the magnetic lens mayhave an equal number of sectors and sector coil windings. Each sectorcoil winding may be configured to drive a magnetic potential of thesector coupled to each sector coil winding, respectively, when a currentis applied to each sector coil winding. Therefore, the magneticpotential of each sector may include the magnetic potential generated bythe current applied to primary coil winding which may affect each sectorsubstantially equally. The magnetic potential of each sector may alsoinclude the magnetic potential generated by the current applied to eachrespective sector coil winding. As such, each sector may have a magneticpotential that may be substantially independent of the magneticpotentials of the other sectors of the magnetic lens. The magnetic fieldthat may be applied to the charged particle beam, therefore, may includethe magnetic potential of the outer pole piece relative to the innerpole piece and the magnetic potential of one or more sectors of themagnetic lens.

In an embodiment, a method for applying a magnetic field to a chargedparticle beam may include directing the charged particle beam through amagnetic lens. Directing the charged particle beam may includepositioning a charged particle beam source configured to generate acharged particle beam in substantial alignment with a first end of themagnetic lens. The charged particle beam may travel from a first end ofthe magnetic lens to a second end of the magnetic lens. The magneticlens may be configured as described in an above embodiment. The methodmay also include applying a first current to a primary coil winding ofthe magnetic lens to drive a magnetic potential of an outer pole pieceof the magnetic lens relative to an inner pole piece of the magneticlens. In addition, the method may include applying a second current toat least one sector coil winding of the magnetic lens to drive amagnetic potential of at least one sector of the magnetic lens.Therefore, the magnetic field that may be applied to the chargedparticle beam may include the magnetic potential of the outer pole piecerelative to the inner pole piece and the magnetic potential of at leastone sector of the magnetic lens.

In an embodiment, a method for focusing a charged particle beam on aspecimen is provided. The specimen may be a semiconductor device thatmay be fabricated using a semiconductor fabrication process.Alternatively, the specimen may be a portion of a semiconductor deviceor another specimen such as a biological sample. The method may includepositioning at least a portion of the specimen in a path of the chargedparticle beam. The charged particle beam may be directed through amagnetic lens from a first end of the magnetic lens to a second end ofthe magnetic lens. The magnetic lens may be configured as described inan above embodiment. Focusing a charged particle beam on a specimen mayalso include applying a first current to a primary coil winding of themagnetic lens to generate a magnetic potential of an outer pole piece ofthe magnetic lens relative to an inner pole piece of the magnetic lens.In addition, focusing a charged particle beam on a specimen may includeapplying a second current to one sector coil winding of the magneticlens to generate a magnetic potential of one or more sectors of theouter pole piece of the magnetic lens. In this manner, the magneticfield that may be applied to the charged particle beam may include themagnetic potential of the outer pole piece relative to the inner polepiece and the magnetic potential of one or more sectors of the magneticlens. The method may also include altering the magnetic potential of theouter pole piece to apply a coarse focus adjustment to the chargedparticle beam. Furthermore, the method may include altering the magneticpotential of one or more sectors to apply a fine focus adjustment to thecharged particle beam.

In an embodiment, a system that may be used to inspect a semiconductordevice is provided. The semiconductor device may be fabricated on asemiconductor substrate using a semiconductor manufacturing process. Thesystem may be a scanning electron microscope that may use an electronbeam to inspect the semiconductor device. The system, however, may beany system that may use a charged particle beam such as a beam ofelectrons or a beam of ions to inspect a semiconductor device. Thesystem may include a charged particle beam source configured to generatea charged particle beam. The system may also include at least onemagnetic lens that may be configured as described herein to apply amagnetic field to the charged particle beam. The magnetic lens may bepositioned along a path of the charged particle beam generated by thecharged particle beam source. As such, the charged particle beam maypass through the magnetic lens from a first end of the magnetic lens toa second end of the magnetic lens. The system may further include astage configured to support at least a portion of the semiconductordevice. The stage may be positioned along the path of the chargedparticle beam such that the charged particle beam may interact with thesemiconductor device. The charged particle beam may interact with thesemiconductor device subsequent to having a magnetic field applied tothe charged particle beam by the magnetic lens.

In an additional embodiment, a method for inspecting a specimen such asa semiconductor device is provided. Inspecting the specimen may includepositioning at least a portion of the specimen on a stage. The stage maybe configured to support the specimen and may be positioned along a pathof the charged particle beam. The method may include generating acharged particle beam. The generated charged particle beam may bedirected through at least one pole piece of a magnetic lens such thatthe charged particle beam may travel from a first end of the magneticlens to a second end of the magnetic lens. The magnetic lens may beconfigured as described herein and may be incorporated into a scanningelectron microscope. The method may further include applying a firstcurrent to a primary coil winding to generate a magnetic potential of anouter pole piece of the magnetic lens relative to an inner pole piece ofthe magnetic lens. In addition, the method may include applying a secondcurrent to a sector coil winding of the magnetic lens to generate amagnetic potential of at least one sector of the outer pole piece.Therefore, the magnetic field applied to the charged particle beam mayinclude the magnetic potential of the outer pole piece and the magneticpotential of one or more sectors of the outer pole piece.

An additional embodiment relates to a computer-implemented method forcontrolling a magnetic field that may be applied to a charged particlebeam. The computer-implemented method may be implemented by controllersoftware that may be executable on a controller computer. The controllercomputer may be coupled to a magnetic lens. The method for controlling amagnetic field may also be implemented by program instructions that maybe incorporated into a carrier medium. The method may include measuringa magnetic field generated within a magnetic lens that may be configuredas described herein. The charged particle beam may be directed throughthe magnetic lens such that the magnetic field of the magnetic lens maybe applied to the charged particle beam.

The method may also include determining a primary current in response tothe measured magnetic field. The primary current may be applied to aprimary coil winding that may be coupled to a pole piece of the magneticlens. In this manner, a magnetic potential of the pole piece may begenerated. In addition, the method may include determining a secondarycurrent in response to the measured magnetic field. The secondarycurrent may be applied to at least one sector coil winding coupled toone sector of the pole piece. As such, a magnetic potential of eachsector of the pole piece may be generated. Therefore, the magnetic fieldapplied to the charged particle beam may include the magnetic fieldstrength of the pole piece of the magnetic lens and the magnetic fieldstrength of one or more sectors of the magnetic lens. The method mayfurther include controlling the applied primary current and the appliedsecondary current. Controlling the applied primary current and theapplied secondary current may be performed while the magnetic lens isbeing used.

In an embodiment, an apparatus configured to control a magnetic fieldstrength of a magnetic lens is provided. The apparatus may include amagnetic sensor disposed within a magnetic field generated by themagnetic lens. The magnetic sensor may be configured to generate anoutput signal that may be responsive to a first magnetic field strengthof the magnetic lens. The apparatus may also include a control circuitthat may be coupled to the magnetic sensor and the magnetic lens. Thecontrol circuit may be configured to receive the output signal from themagnetic sensor and an input signal that may be responsive to apredetermined magnetic field strength. A manually-controlled operatingsystem or a computer-controlled operating system may be coupled to theapparatus and may be configured to generated the input signal. Inaddition, the control circuit may be configured to generate a controlsignal that may be responsive to the output signal and the input signal.Furthermore, the control circuit may be configured to apply a current tothe magnetic lens. The current may be responsive to the control signal.Therefore, the applied current may be effective to generate a secondmagnetic field strength within the magnetic lens. The second magneticfield strength may be closer to the predetermined magnetic fieldstrength than the first magnetic field strength. In addition, the secondmagnetic field strength may be approximately equal to the predeterminedmagnetic field strength.

In an embodiment, a magnetic lens such as a magnetic lens as describedherein may be configured to apply a magnetic field to a charged particlebeam. The magnetic lens may be positioned along a path of the chargedparticle beam such that the charged particle beam may pass through themagnetic lens. The charged particle beam may be an electron beam or anion beam. As such, the apparatus as described herein and the magneticlens may be coupled to a system such as a scanning electron microscope.The system may also include a charged particle beam source that may beused to produce the charged particle beam. In addition, the system mayfurther include a stage configured to support at least a portion of aspecimen. The stage may also be positioned along a path of the chargedparticle beam such that the charged particle beam may interact with thespecimen. The apparatus and the magnetic lens may, therefore, be used toinspect a specimen such as at least a portion of a semiconductor device,which may be fabricated by using a semiconductor fabrication process.

In an additional embodiment, a method for controlling a magnetic fieldstrength of a magnetic lens is provided. The method may be performedsubstantially continuously or substantially intermittently. The methodmay include generating an output signal in response to a first magneticfield strength generated by the magnetic lens. The method may alsoinclude receiving an input signal. The input signal may be generated inresponse to a predetermined magnetic field strength. Furthermore, themethod may include generating a control signal in response to the outputsignal and the input signal. Additionally, the method may also includeapplying a current to the magnetic lens. The applied current may beapplied in response to the control signal.

In a further embodiment, a method for inspecting a specimen is provided.The specimen may include at least a portion of a semiconductor device.The specimen may be positioned along a path of the charged particle beamby a stage configured to support the specimen. The method may includegenerating a magnetic field by a magnetic lens and applying the magneticfield to a charged particle beam. Applying the magnetic field to thecharged particle beam may include directing the charged particle beamthrough the magnetic lens. The charged particle beam may be an electronbeam or an ion beam. The charged particle beam may be generated by usinga charged particle beam source. Therefore, the magnetic lens may becoupled to a system such as a scanning electron microscope. In addition,the method may include controlling a magnetic field strength of themagnetic lens as described in the above embodiment.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects and advantages of the invention will become apparent uponreading the following detailed description and upon reference to theaccompanying drawings in which:

FIG. 1 depicts a perspective view of an embodiment of a magnetic lensincluding a pole piece having eight sectors and eight slots;

FIG. 2 depicts a side view of an embodiment of a pole piece that haseight sectors and eight slots;

FIG. 3 depicts a schematic view of an embodiment of a system thatincludes at least one magnetic lens that has at least one pole piecehaving at least two sectors;

FIG. 4 depicts a flow chart of an embodiment of a method for applying amagnetic field to a charged particle beam using a magnetic lens thatincludes at least one pole piece having at least two sectors;

FIG. 5 depicts a schematic view of an embodiment of an apparatusconfigured to control a magnetic field of a magnetic lens;

FIG. 6 a depicts a flow chart of an embodiment of a method forcontrolling a magnetic field of a magnetic lens in which a magneticsensor is disposed within a magnetic fringe field area of a magneticlens;

FIG. 6 b depicts a schematic view of an embodiment of a magnetic lens inwhich a magnetic sensor is disposed within a cavity interposed betweenan outer pole piece of a magnetic lens and an inner pole piece of themagnetic lens;

FIG. 6 c depicts a schematic view of an embodiment of a magnetic lens inwhich a magnetic sensor is disposed within an inner pole piece of themagnetic lens;

FIG. 7 depicts a plot of the hysteresis of a magnetic lens using acurrent feedback control apparatus; and

FIG. 8 depicts a plot of the hysteresis of a magnetic lens using amagnetic field feedback control apparatus.

While the invention is susceptible to various modifications andalternative forms, specific embodiments thereof are shown by way ofexample in the drawings and will herein be described in detail. Itshould be understood, however, that the drawings and detaileddescription thereto are not intended to limit the invention to theparticular form disclosed, but on the contrary, the intention is tocover all modifications, equivalents and alternatives falling within thespirit and scope of the present invention as defined by the appendedclaims.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Turning now to the drawings, FIG. 1 illustrates a perspective view of anembodiment of a magnetic lens. As used herein, a “magnetic lens” isgenerally defined as a magnetic circuit configured to apply a magneticfield to a charged particle beam. Magnetic lens 10 may have outer polepiece 12 coupled to inner pole piece 14. FIG. 2 further illustrates aside view of an embodiment of outer pole piece 12 of magnetic lens 10.Further description of magnetic lens 10 and outer pole piece 12 will bemade with respect to both FIGS. 1 and 2. A charged particle beam (notshown) may be configured to travel through the magnetic lens along axis16 from first end 18 of magnetic lens 10 to second end 20 of magneticlens 10. The charged particle beam may include an electron beam or anion beam. First end 18 and second end 20 of magnetic lens 10 may formtwo ends of a channel through the magnetic lens along axis 16 throughwhich the charged particle beam may pass during operation. The chargedparticle beam may also, therefore, effectively travel through the outerpole piece along axis 16 from a first end 22 of the outer pole piece tosecond end 24 of the outer pole piece. Axis 16 of outer pole piece 12may be substantially the same as axis 16 of magnetic lens 10.

Magnetic lens 10 may be configured to apply a magnetic field to thecharged particle beam in order to alter and/or control the path of thecharged particle beam through the magnetic lens. As such, magnetic lens10 may be configured to operate as a magnetic objective lens that may beincorporated into any device that uses a charged particle beam duringoperation. Examples of such devices include, but are not limited to,scanning electron microscopes, tunneling electron microscopes, e-beamlithography tools, and focused ion beam inspection tools. Magnetic lens10 may, however, be incorporated into any device that uses an appliedmagnetic field to alter the direction of a charged particle beam.

The two pole pieces, 12 and 14, may be coupled such that a negligibleair gap is formed between the two pole pieces. Minimizing the air gapbetween the two pole pieces may be advantageous to reduce gap deflectionflux leakage, or magnetic flux that may escape through an air gapbetween the pole pieces. Such air gaps may induce eddy currents in theouter pole pieces and the lens coil. In addition, thin circularshielding rings of a magnetic material may be placed in a gap betweenthe inner pole piece and the outer pole piece of the magnetic lens tofurther minimize gap deflection flux leakage.

Outer pole piece 12 and inner pole piece 14 may be formed from amagnetically “soft” material such as ferrite, iron, holium metal, orother suitable material having a high magnetic permeability with minimumhysteresis. Inner pole piece 14 may be coupled to outer pole piece 12 byscrews 26, or other suitable connecting devices. The connecting devicemay be made of non-magnetic materials. Alternatively, the two polepieces may be connected by a physically soft material such as asilicone-based polymeric material that may flex radially to accommodatedifferential thermal expansion between the materials of the magneticlens. Connecting the pole pieces using a physically soft connectingmaterial, which is non-magnetic, may reduce fluctuations in the magneticfield of the magnetic lens as the temperature of the magnetic lensvaries due to heat generated by the magnetic lens during operation. Thevariations in temperature due to heat generated during operation mayalso be reduced by cooling the magnetic lens through evaporation coolingor by forced cooling of the lens by circulating a cooling fluidproximate the magnetic lens.

Exterior surface 28 of outer pole piece 12 may have a substantiallyconical shape tapering outward from second end 24 of the pole piece. Theconical shape of exterior surface 28 may be advantageous, for example,to minimize the distance between the bore, depicted as the opening insecond end 20, of magnetic lens 10 and a specimen along a path of thecharged particle beam. Therefore, the distance that a charged particlebeam travels external to the applied magnetic field may be reduced, andeven minimized. Consequently, the charged particle beam that impingesupon a specimen may more accurately reflect the characteristics of thebeam within the magnetic lens. Furthermore, the conical shape ofexterior surface 28 also increases the amount of space which isavailable for other elements coupled to a system with the magnetic lens.In a scanning electron microscope, for example, a stage configured tosupport a specimen may be positioned in the path of the electron beamproximal second end 20 of magnetic lens 10. The stage may also be tiltedto provide different angles for scanning the charged particle beamacross the sample. Therefore, a magnetic lens having a conical shape mayreduce the restrictions on the mechanical devices that may be used in ascanning electron microscope and the restrictions to the functioning ofthese mechanical devices. Exterior surface 28, however, may also have asubstantially cylindrical shape across outer pole piece 12.Additionally, exterior surface 28 may also have a partially conicalshape and a partially cylindrical shape. Similarly, inner pole piece 14may also have an exterior shape which is conical, cylindrical, or acombination of the two shapes. The exterior shapes of both pole piecesshould be designed, however, to facilitate connection of the pole pieceswhile minimizing any gap which may be formed between the pole pieces.

Portion 30 of outer pole piece 12 may have a number of sectors 32 and anumber of slots 34. Each sector 32 may be defined as the portion 30 ofouter pole piece 12 disposed between lateral boundaries 36 of two slots34. Slots 34 may be configured to extend through substantially theentire thickness of outer pole piece 12, from exterior surface 28 ofouter pole piece 12 to an interior surface (not shown) of outer polepiece 12. Slots 34, however, may also extend partially into outer polepiece 12 from exterior surface 28. Furthermore, first portion 38 ofslots 34 may extend through substantially the entire thickness of outerpole piece 12, while second portion 40 of slots 34 may partially extendinto outer pole piece 12. As such, the second potion of slots 34 mayprovide a space for coil windings configured to encircle each sectorthrough adjacent slots.

Outer pole piece 12 may have eight sectors 32 and eight slots 34. Thenumber of sectors on the pole piece may be larger or smaller, however,depending upon the desired performance of the magnetic lens. Forexample, a pole piece having only two sectors and two slots maysubstantially enhance the performance of the magnetic lens in comparisonto the performance of a magnetic lens that does not have sectors.Sectors 32 and slots 34 may also be formed in inner pole piece 14 (notshown). In this manner, outer pole piece 12, inner pole piece 14 or bothpole pieces of magnetic lens 10 may have a number of sectors and slots.Sectors 32 and slots 34 may be arranged around substantially the entireportion 30 of outer pole piece 12. Alternatively, sectors 32 and slots34 may be arranged around only a fraction of portion 30 of outer polepiece 12.

Slots 34 may be spaced evenly around outer pole piece 12. For example,if outer pole piece 12 has eight slots, then a slot may be located everyforty five degrees on outer pole piece 12. Alternatively, slots 34 mayalso have an uneven spatial arrangement around outer pole piece 12.Lateral boundaries 36 surrounding slots 34 define a lateral length 42 ofeach slot. In this manner, each slot 34 may have substantially the samelateral length 42. Lateral length 42 of each slot 34 may also besubstantially smaller than a lateral length 44 of sectors 32. Laterallength 44 of sectors 32 may be defined as a length of each sector 32disposed between two slots 34, and the lateral length 44 of each sector32 may also be substantially equal. Alternatively, lateral length 44 ofeach sector 32 may vary from sector to sector.

Slots 34 may extend vertical length 46 from second end 24 of outer polepiece 12 across portion 30 of outer pole piece 12. Vertical length 46 ofslots 34 may be larger or smaller depending on the designcharacteristics of magnetic lens 10. Each slot 34 may have substantiallythe same vertical length 46, and vertical length 46 may be substantiallyequal to vertical length 48 of each sector 42. Vertical length 48 ofsectors 32 may also be defined as a length of sector 32 extending acrossportion 30 of outer pole piece 12 from second end 24 of the pole piece.Therefore, each sector 32 may also have a vertical length 48 that may besubstantially equal to vertical length 48 of the other sectors.

A primary coil winding (as shown in FIG. 6 a as primary coil winding 134disposed within magnetic lens 110) may also be disposed within magneticlens 10 and may be interposed between outer pole piece 12 and inner polepiece 14. As such, the primary coil winding may be disposed within acavity formed between outer pole piece 12 and inner pole piece 14. Theprimary coil winding may be configured to generate a magnetic potential,Φ_(o), of outer pole piece 12 relative to a magnetic potential of innerpole piece 14 when a current, I_(o), is applied to the primary coilwinding. The primary coil winding may be a number of turns of aconductive wire. The number of turns of the conductive wire may beapproximately 100 to approximately 1000. The number of turns of the coilmay be larger or smaller, however, depending on, for example, theintended use for the magnetic lens. The primary coil winding may beformed of an electrically conductive wire, such as copper, anodizedaluminum, or other suitable material. The primary coil winding may alsobe cooled by various means during operation in order to reduce the heatgenerated by the coil by evaporation cooling or by forced coolinginvolving circulating a cooling fluid proximate the primary windingcoil.

Magnetic lens 10 may also have a number of sector coil windings 50. Eachsector coil winding 50 may be coupled to one sector 32 of magnetic lens10. Each sector coil winding 50 may be wound through opening formed inouter pole piece 12 by two adjacent slots 34. As such, each sector coilwinding 50 may be arranged to encircle one sector 32 of the magneticlens. The sector coil winding may, therefore, encircle a portion of thesector or substantially the entire sector and may also include a numberof turns of a conductive wire. The number of turns of the conductivewire may be configured to encircle the sector in a directionsubstantially perpendicular to the path of the charged particle beamthrough the magnetic lens. The number of turns for each sector coilwinding may be smaller than the number of turns for the primary coilwinding. The sector coil winding may also be formed of an electricallyconductive material and may also be cooled during operation as describedabove.

The magnetic potential of each sector 32 may be first established by themagnetic potential Φ_(o), of outer pole piece 12 generated by theprimary coil winding. Therefore, when a current is applied to theprimary coil winding, a magnetic potential of outer pole piece 12relative to inner pole piece 14 may be generated, which may be appliedapproximately equally to each sector 32 of magnetic lens 10. Current,I_(i), may also be passed through each sector coil winding 50 togenerate a magnetic potential, Φ_(i), for each sector 32 of magneticlens 10. A magnetic potential may, therefore, also be individuallygenerated on each sector 32 of magnetic lens 10. The resulting magneticpotential on the i^(th) sector, Φ_(i), may, therefore, be approximatelyequal to the sum of the magnetic potential resulting from the currentapplied to the primary coil winding, Φ_(o), and the magnetic potentialresulting from the current applied to each sector coil winding, ΔΦ₁, orΦ_(i)=Φ_(o)+ΔΦ_(i), where ΔΦ_(i) is approximately proportional to thecurrent applied to the sector winding, I_(i). As such, the magneticfield which may be applied to the charged particle beam may include themagnetic potential of outer pole piece 12 relative to inner pole piece14 and a magnetic potential of each sector 32 of outer pole piece 14.

In addition, current I_(i) passed through each sector coil winding 50may include the zero, first, and second order harmonics which may beexpressed by the following equation: I_(i)=A*cos (0*i)+B*cos(π/4*i+β)+C*cos (π/2*i+γ). The quantity “A” may represent the magnitudeof the fine adjustment of the focus strength of the magnetic lens whichis applied to the magnetic lens by the magnetic potential generated bythe i^(th) sector. The quantity “B” may represent the amount of magneticaxis displacement in the magnetic lens which is applied to the magneticlens by the magnetic potential generated by the i^(th) sector, and “β”may represent the direction of the magnetic axis displacement. Thequantity “C” may represent the strength of the stigmation of themagnetic lens which is applied to the magnetic lens by the magneticpotential generated by the i^(th) sector, and “γ” may represent thedirection of the stigmation axis. Therefore, the magnetic potentialgenerated on each sector of the pole piece may be used to adjust thefocus strength of the magnetic lens, the amount of magnetic axisdisplacement of the magnetic lens, and/or the strength of stigmation ofthe magnetic lens. In this manner, a current may be applied to eachsector coil winding of the magnetic lens to alter the magnetic potentialapplied to the charged particle beam.

The performance of a magnetic lens may be substantially enhanced byincorporating sectors on at least one pole piece of the magnetic lens.For example, altering the magnetic potential of the magnetic lens bygenerating individual magnetic potentials for each sector that arecoupled to separate sector coil windings may enable the symmetry ofmagnetic lenses to be broken in a controlled fashion. The potentialadvantages of such a magnetic lens include improved capability to makefine adjustments to the focusing strength of the magnetic field.Furthermore, a sectored magnetic lens may enable displacing the magneticaxis of the magnetic lens that may be useful for the purpose of aligningthe lens axis with an off-axis charged particle beam in order tominimize aberrations in the charged particle beam. Displacing themagnetic axis may also allow any errors in the symmetry of the magneticlens to be corrected in order to obtain a symmetric magnetic field.Being able to correct the symmetry of a magnetic lens may further reducethe rejection rate of magnetic lenses associated with magnetic lensmanufacturing. Additionally, displacing the magnetic axis may be usefulin producing a deflection of the charged particle beam to affect thetrajectory of the charged particle beam. In this manner, a sectoredmagnetic lens may provide improved control over the landing position andthe landing angle of the charged particle beam on a specimen or adetector. A stigmator field may also be produced using a magnetic lenshaving sectors on at least one pole piece, which may correct for otherstigmating forces on the charged particle beam. Furthermore, because asectored magnetic lens may be operated as a multi-pole device withoutinserting additional devices within the magnetic lens, the sectoredmagnetic lens also provides the benefits of a multi-pole device whilemaintaining a co-linear path for the charged particle beam. Therefore,an advanced magnetic lens design is provided without increasing thecomplexity of the overall optical design of a system in which the lensis incorporated.

A magnetic lens having at least one pole piece which includes at leasttwo sectors may also be incorporated into a system configured to inspecta specimen such as a semiconductor device. Inspecting semiconductordevices is an important step in manufacturing a semiconductor device.Inspection of semiconductor devices may usually be performed to controland improve fabrication processes. Inspection may be performed afterindividual processes have been performed or after the entire device hasbeen fabricated. In addition to semiconductor devices, inspection ofsubstantially transparent reticles may also be performed duringsemiconductor manufacturing. Reticles may be used in lithography totransfer a pattern to a resist on a semiconductor substrate. Therefore,a defect in or on a reticle will also be transferred to thesemiconductor device. As such, careful inspection of the reticle mayusually be performed during manufacture of the reticle itself and duringsubsequent use in semiconductor device manufacturing.

As the dimensions of devices shrink, it is becoming increasinglydifficult to successfully fabricate semiconductor devices. Therefore, itis also becoming increasingly important to monitor, control, and improvethe performance of semiconductor fabrication processes. Analysis offabrication processes, such as lithography and etch, may typically beperformed by generating an image of the device and analyzing thesemiconductor process by observing the image quality and measuringcritical dimensions of features of the device. Due to reductions infeature sizes of a semiconductor device, an inspection tool thatutilizes a charged particle beam to generate an image of a semiconductordevice may generally be used to inspect the manufactured devices.Examples of such devices include scanning electron microscopes,tunneling electron microscopes and focused ion beam inspection devices.

FIG. 3 illustrates an embodiment of a system configured to inspect aspecimen. System 54 may include charged particle beam source 56, such asan ion beam or an electron beam, configured to generate charged particlebeam 58. Charged particle beam source 56 may be any known in the artsuch as a cold field emission source or a thermal field emission source.System 54 may also include several positioning devices 60 located alongthe path of charged particle beam 58 and configured to direct thecharged particle beam to a focusing device. Positioning devices 60 mayinclude electrostatic or electromagnetic deflectors, beam-limitingapertures, Wien filters, and magnetic condenser lenses. Appropriatepositioning devices, however, may vary depending upon, for example, theintended application for the system and may include any known in theart. The positioning devices may be configured to alter the path of thecharged particle beam such that the charged particle beam may besubstantially aligned with a first end of the magnetic lens. Inaddition, the positioning devices may also be configured to apply aninitial correction to the charged particle beam to reduce effects suchas chromatic aberrations, and/or dispersion in the energy of the chargedparticle beam.

System 54 may include at least one magnetic lens 62. Magnetic lens 62may be positioned such that charged particle beam 58 may enter themagnetic lens at a first end of the magnetic lens and may exit themagnetic lens at a second end of the magnetic lens. A substantiallyco-linear void in the magnetic lens from the first end to the second endmay provide an appropriate path for the charged particle beam.Therefore, magnetic lens 62 may be configured to apply a magnetic fieldto charged particle beam 58 as the charged particle beam travels throughthe magnetic lens. At least one pole piece of magnetic lens 62 may haveat least two sectors and at least two slots. The sectors may be disposedbetween lateral boundaries of the slots in the pole piece. Magnetic lens62 may have a primary coil winding disposed within the magnetic lens.The magnetic lens may also include a number of secondary coil windings,and each sector coil winding may be coupled to one sector of the polepiece. As such, the primary coil winding may be configured to generate amagnetic potential of the outer pole piece relative to the inner polepiece, and each secondary coil winding may be configured to generate amagnetic potential of one sector of the pole piece. The magnetic fieldapplied to charged particle beam 58 by magnetic lens 62, therefore, mayinclude the magnetic potential of the outer pole piece relative to theinner pole piece and the magnetic potential of at least one sector ofthe pole piece. The magnetic lens may be further configured as describedherein. The magnetic lens may be also configured to operate as amagnetic objective lens to focus the charged particle beam onto asemiconductor device. Focusing the charged particle beam may includereducing aberrations in the charged particle beam and reducing thediameter of the charged particle beam to a spot size which isappropriate for imaging a semiconductor device.

System 54 may further include stage 64 configured to support at least aportion of specimen 66, which may include at least a portion of asemiconductor device formed on a semiconductor substrate, or a productwafer. Stage 64 may be positioned along the path of charged particlebeam 58. The stage may be any mechanical device known in the art. Forexample, stage 64 may include a holder configured to engage a stud. Thestud may include a horizontally flat upper surface configured to supportthe specimen and a post that may be substantially vertical to the uppersurface. Semiconductor device 66 may be attached to a stud using anadhesive. The holder may be positioned in the path of the chargedparticle beam by additional mechanical devices and may include voidsconfigured to engage the post of the stud. As such, the stud may be alsobe positioned in the path of charged particle beam when the stud isengaged within the holder. Alternatively, stage 64 may have a flatsurface which may support specimen 66. The flat surface of the stage mayalso have small holes through which a vacuum source may be connected. Inthis manner, the stage may be configured to engage the specimen using avacuum which may be generated by the vacuum source during operation. Thestage may also be positioned in the path of the charged particle beammanually or by a mechanical device controlled by a controller computer.

Specimen 66 may also be positioned in the path of charged particle beam58 such that the charged particle beam may interact with the specimen togenerate a secondary beam of charged particles 68. Secondary beam ofcharged particles 68 may include secondary charged particles, which mayemanate from recesses of the specimen, back-scattered charged particles,which may emanate from the surface of the specimen, and transmittedelectrons, which may pass through the specimen, such as a substantiallytransparent reticle. System 54 may also include detector 70 or aplurality of detectors and at least one device 72 configured to directthe secondary beam of electrons to the detectors. Device 72 may include,for example, a Wien filter. The Wien filter, or other suitable device,may be configured to alter the path of the secondary electrons withoutaffecting the path of the charged particle beam which is being directedto the semiconductor device. Detector 70 may be a Schottky solid statebarrier detector, or other suitable detectors, and may also be coupledto operating system 74, which may be incorporated into the system.Operating system 74 may be configured to receive a signal from detector70, to analyze the signal, and to generate information aboutcharacteristics of specimen 66 such as feature size. Operating system 74may also be coupled to imaging device 76, which may be a cathode raytube. In this manner, system 54 may also be configured to generate imageprofile characteristics of specimen 66. The characteristics of thedevice may be used to control and/or alter a process, which was used tofabricate the specimen. Additional features and devices that may also beincorporated into the system are illustrated, for example, in U.S. Pat.No. 4,928,010 to Saito et al., U.S. Pat. No. 5,241,176 to Yonezawa, U.S.Pat. No. 5,502,306 to Meisburger et al., U.S. Pat. No. 5,578,821 toMeisburger et al., U.S. Pat. No. 5,665,968 to Meisburger et al., U.S.Pat. No. 5,717,204 to Meisburger et al., U.S. Pat. No. 5,869,833 toRichardson et al., U.S. Pat. No. 5,872,358 to Todokora et al., and U.S.Pat. No. 5,973,323 to Adle and are incorporated by reference as if fullyset forth herein.

FIG. 4 illustrates an embodiment of a method for applying a magneticfield to a charged particle beam. As shown in step 78, a chargedparticle beam may be generated by supplying power to a charged particlebeam source. The charged particle beam source may be any known in theart including a cold field emission source or a thermal field emissionsource. Additionally, the charged particle beam may include a beam ofelectrons or ions. The charged particle beam source may be disposedwithin an optical column, which may be held under vacuum conditionsduring operation. As shown in step 80, the charged particle beam may bedirected to a first end of the magnetic lens by applying electrostaticor magnetic fields to the charged particle beam. For example, prior totraveling through the magnetic lens, the charged particle beam may bepassed through several positioning devices located along the path of thecharged particle beam. The positioning devices and the magnetic lens mayalso be incorporated into the optical column containing the chargedparticle beam source. As such, the positioning devices and the magneticlens may be operated while the optical column is under vacuumconditions. Positioning devices may include electrostatic orelectromagnetic deflectors, beam-limiting apertures, Wien filters andmagnetic condenser lenses. The positioning devices may alter the path ofthe charged particle beam to substantially align the charged particlebeam with a first end of the magnetic lens. In addition, the positioningdevice may also apply an initial correction to the charged particle beamto reduce effects such as chromatic aberrations, or dispersion in theenergy of the charged particle beam.

The charged particle beam may be directed to the magnetic lens such thatthe charged particle beam may travel substantially through the magneticlens which includes at least one pole piece. The charged particle beammay enter the magnetic lens at a first end of the magnetic lens and mayexit the magnetic lens at a second end of the magnetic lens. Themagnetic lens may be configured as described herein.

As shown in step 82, a magnetic field may be generated within themagnetic lens by driving a magnetic potential of the pole piece. Forexample, a first current may be applied to a primary coil windingcoupled to the magnetic lens. A second current may also be applied to atleast one sector coil winding coupled to a sector of the magnetic lens.In this manner, a magnetic potential of at least one sector of themagnetic lens may also be generated. Therefore, the magnetic fieldapplied to the charged particle beam traveling through the magnetic lensmay include the magnetic potential of the outer pole piece relative tothe inner pole piece and the magnetic potential of at least one sector.The magnetic potential of at least one sector of the pole piece mayalter the focus strength of the magnetic lens, the magnetic axisdisplacement of the magnetic lens, and/or the strength of stigmation ofthe magnetic lens. In this manner, the magnetic lens may have anenhanced capability to reduce aberrations in the charged particle beam.

In an additional embodiment, the method, as shown in FIG. 4, may includeadditional steps to focus a charged particle beam on a specimen. Thespecimen may include a semiconductor device, a feature or a level of asemiconductor device, or other suitable specimen such as a substantiallytransparent reticle configured to transfer a pattern to a resist inlithography, a photoresist layer suitable for e-beam lithography, or abiological sample. The specimen may also be an entire semiconductorsubstrate that has been processed using a semiconductor fabricationprocess or a portion of the semiconductor substrate. For example, aportion of a semiconductor substrate may be formed by cutting thesubstrate into segments at appropriate positions on the substrate. Assuch, a cross-sectional portion of semiconductor device, or othersemiconductor feature of interest, may be exposed to the chargedparticle beam. An appropriate portion of the specimen may also be largeror smaller depending on the capability of the device coupled to thecharged particle beam. As shown in step 84, a specimen such as asemiconductor device may be fabricated on a semiconductor substrateusing a semiconductor manufacturing process. The semiconductormanufacturing process may be any process known in the art ofsemiconductor manufacturing, such as lithography, etch, ionimplantation, deposition, chemical mechanical polishing, and/or plating.

At least a portion of the specimen may be positioned in the path of thecharged particle beam prior to generating the charged particle beam, asshown in step 86. The specimen may be positioned in the path of thecharged particle beam by placing the specimen on a stage. The stage maybe located substantially within the optical column or proximal theoptical column such that the stage and the specimen are also undervacuum conditions during operation. The stage may be configured tosupport the specimen such that a position of the specimen in the path ofthe charged particle beam may be maintained throughout a process. Forexample, the specimen may be attached to a stud using an adhesive. Thestud may be configured as described herein. Alternatively, the stage mayhave a flat surface configured to support substantially the entirespecimen such as a semiconductor substrate or product wafer. The flatsurface of the stage may also have holes through which a vacuum may bepulled in order to retain the position of the specimen in the path ofthe charged particle beam. The stage, however, may also be any suitablemechanical device known in the art. The stage may also be positioned inthe path of the charged particle beam manually or by a mechanical devicethat may be controlled by a controller computer.

In an additional embodiment, the method, as shown in FIG. 4, may includeadditional steps to detect at least one secondary beam of chargedparticles, as shown in step 88. The secondary beam of charged particlesmay be produced as a result of interactions between the charged particlebeam and the specimen. The secondary beam of charged particles mayinclude secondary charged particles, back-scattered charged particles,and/or transmitted charged particles. The secondary beam of electronsmay be directed back through the magnetic lens. A device such as a Wienfilter, which may generate an electrostatic field or a combination ofelectrostatic and magnetic fields, may be used to direct the secondarybeam of charged particles to a detector or a plurality of detectors. Assuch, the Wien filter direct the secondary beam to a detector at a highangle of incidence from the original path of the secondary beam. TheWien filter, however, may apply the field to the secondary beam withoutaffecting the trajectory of the charged particle beam, which may alsopass through the Wien filter. The secondary beam of charged particlesmay then be detected by a Schottky solid state barrier detector, orother suitable detector. The detector may be configured to detect thesecondary beam of charged particles and to generate a signal responsiveto the secondary beam.

In an embodiment, an image of the specimen may be generated subsequentto detecting the secondary beam of charged particles, as shown in step90. An operating system coupled to the detector may receive a signalgenerated by at least one detector. The signal from the detector may beresponsive to the secondary beam of charged particles. The operatingsystem may include a computer such as a personal computer or a mainframecomputer, which may have an appropriate software package to perform anappropriate set of operations on the signal from the detector. As such,the operating system may analyze the signal from the detector andgenerate an output signal representative of an image of the specimen. Animaging system such as a cathode ray tube, which may be coupled to theoperating system, may receive the output signal from the operatingsystem and generate an image of the specimen. The image may be used toanalyze physical characteristics of the specimen such as feature sizeand vertical profile quality. The physical characteristics of the devicemay then also be used to control and/or improve the a process which wasused to fabricate the specimen.

In a further embodiment, the method as illustrated in FIG. 4 may alsoinclude applying a coarse focus adjustment to the charged particle beam,as shown in step 92. An initial magnetic potential may be generatedwithin the magnetic lens by applying a current to a primary coil windingof the magnetic lens. The initial magnetic potential may also begenerated within the magnetic lens by further applying a current tosector coil winding coupled to a sector of the magnetic lens. Theinitial magnetic potential may be a function of the magnetic potentialgenerated by the primary coil winding and the magnetic potentialgenerated by the sector coil winding as described above. The initialmagnetic potential may adequately focus, or reduce aberrations in, thecharged particle beam on a specimen in order to produce an image of thespecimen. The image of the specimen may be generated as described in theabove embodiments. For example, the image of the specimen may begenerated by detecting at least one secondary beam of charged particleswhich may be result from interactions between the charged particle beamand the specimen. The image of the specimen may be observed using aimaging device such as a cathode ray tube.

The clarity of the image of the specimen may generally be dependent onthe aberrations such as chromatic aberration and stigmation in thecharged particle beam, which interacts with the specimen. Therefore, theinitial magnetic potential may not sufficiently reduce aberrations in,or focus, the charged particle beam in order to obtain an adequate imageof the specimen. In step 92, a coarse focus adjustment may be applied tothe charged particle beam by altering the magnetic potential within themagnetic lens. The coarse focus adjustment may reduce aberrations in thecharged particle beam, which are present despite the initial magneticfield. Altering the magnetic potential within the magnetic lens mayinclude altering and/or controlling the current to the primary coilwinding and the current to at least one sector coil winding using amanually-controlled device or a controller computer. The magneticpotential within the magnetic lens may also be altered by alteringand/or controlling the current being applied to several sector coilwindings using a manually-controlled device or a controller computer.

The manually-controlled device and the controller computer may both becoupled to the magnetic lens. For example, an operator may observe theimage of the specimen generated by the initial magnetic potential of themagnetic field and may manually adjust the current being applied to theprimary coil winding and the current being applied to at least one ofthe sector coil windings by using a manually-controlled device. Themanually-controlled device may include a dial, or other suitable device,which may be coupled to the power supply of the primary coil winding ora sector coil winding. Alternatively, a controller computer may analyzea gray-scale image of the specimen generated by the initial magneticpotential of the magnetic field. The controller computer may alter theprimary current applied to the primary coil winding and the currentapplied to at least one sector coil winding using a set of predefinedmathematical equations.

After the coarse focus adjustment, the current being applied to theprimary coil winding may generally remain constant. For example, thecoarse focus adjustment may adequately reduce aberrations in the chargedparticle beam such that only fine adjustments to the charged particlebeam may be required. Additionally, maintaining a constant current tothe primary coil winding may simplify further operation of the magneticlens. In step 94, a fine focus adjustment may also be applied to thecharged particle beam by further altering the magnetic potential withinthe magnetic lens. Further altering the magnetic potential within themagnetic lens may include altering and controlling the current to atleast one sector coil winding, or multiple sector coil windings, using amanually-controlled device or a controller computer. Themanually-controlled device and the controller computer may both becoupled to the magnetic lens. As such, further altering of the magneticpotential within the magnetic lens may be performed using themanually-controlled device or the controller computer, as described instep 92. For example, an operator may manually adjust the current beingapplied to at least one of the sector coil windings by using amanually-controlled device. As described above, the manually-controlleddevice may include a dial, or other suitable device, which may becoupled to the power supply of at least one sector coil winding.Additional dials, or other suitable devices, may also be coupled toadditional sector coil windings such that the current being applied toeach sector coil winding may be controlled individually. Alternatively,a controller computer may automatically adjust the current being appliedto at least one sector coil winding by using a set of predefinedmathematical equations. The fine focus adjustment to the chargedparticle beam may include an adjustment to the magnitude of a focusstrength of the magnetic lens, an adjustment to an amount of magneticaxis displacement of the magnetic lens, and/or an adjustment to astrength of stigmation of the magnetic lens. The fine focus adjustmentmay, therefore, generate a magnetic potential within the magnetic lensto reduce aberrations in the charged particle beam such that an adequateimage of the specimen may be generated.

Furthermore, the methods for applying a magnetic field to a chargedparticle beam may be integrated into a controller for a magnetic lens.The controller may by a computer system configured to operate softwareto control the operation of the magnetic lens. The computer system mayinclude a memory medium on which computer programs for operating themagnetic lens and performing calculations related to the data collected.The term “memory medium” is intended to include an installation medium,e.g., a CD-ROM, or floppy disks, a computer system memory such as DRAM,SRAM, EDO RAM, Rambus RAM, etc., or a non-volatile memory such as amagnetic media, e.g., a hard drive, or optical storage. The memorymedium may include other types of memory as well, or combinationsthereof. In addition, the memory medium may be located in a firstcomputer in which the programs are executed, or may be located in asecond different computer that connects to the first computer over anetwork. In the latter instance, the second computer provides theprogram instructions to the first computer for execution. Also, thecomputer system may take various forms, including a personal computersystem, mainframe computer system, workstation, network appliance,Internet appliance, personal digital assistant (PDA), television systemor other device. In general, the term “computer system” may be broadlydefined to encompass any device having a processor that executesinstructions from a memory medium.

The memory medium preferably stores a software program for the operationof the magnetic lens. The software program may be implemented in any ofvarious ways, including procedure-based techniques, component-basedtechniques, and/or object-oriented techniques, among others. Forexample, the software program may be implemented using ActiveX controls,C++ objects, JavaBeans, Microsoft Foundation Classes (MFC), or othertechnologies or methodologies, as desired. A CPU, such as the host CPU,executing code and data from the memory medium includes a deviceconfigured to create and execute the software program according to themethods described above.

Various embodiments further include receiving or storing instructionsand/or data implemented in accordance with the foregoing descriptionupon a carrier medium. Suitable carrier media include memory media orstorage media such as magnetic or optical media, e.g., a disk or CD-ROM,as well as signals such as electrical, electromagnetic, or digitalsignals, conveyed via a communication medium such as networks and/or awireless link.

The software for the magnetic lens may be used to control the magneticfield applied to a charged particle beam. Preferably, predefinedmathematical equations that describe the relationships between themagnetic potentials of the pole piece, the sectors of the pole piece andthe current applied to each coil winding of the magnetic lens may beincorporated into the software. The software may be configured tomeasure a magnetic field generated within a magnetic lens. The magneticlens may be configured as described herein. The software may also beconfigured to determine a primary current in response to the measuredmagnetic field, which may be applied to a primary coil winding coupledto a pole piece of the magnetic lens. The software may be furtherconfigured to determine a secondary current in response to the measuredmagnetic field, which may be applied to at least one secondary coilwinding coupled to a sector of the outer pole piece. Additionally, thesoftware may be configured to control the applied primary current andthe applied secondary current. In this manner, the software may beconfigured to maintain a predetermined magnetic field within themagnetic lens or to correct for magnetic field drift within the magneticfield of the magnetic lens. Magnetic field drift may occur due to theperformance of the electron beam source, hysteresis effects within themagnetic lens, temperature dependent properties of the magnetic lens,and/or drift caused by sources external to the magnetic lens.

FIG. 5 illustrates an embodiment of an apparatus configured to control amagnetic field strength, or the magnetic flux density, of a magneticlens. Apparatus 96 may include magnetic sensor 98 that may be disposedwithin a magnetic field generated by magnetic lens 100. Apparatus 96 mayalso include control circuit 102 which may be coupled to magnetic sensor98 and magnetic lens 100. The apparatus may be configured tocontinuously control the magnetic field strength of the magnetic lens.Alternatively, the apparatus may be configured to intermittently controlthe magnetic field strength of the magnetic lens. For example, theapparatus may be configured to alter and/or control the magnetic fieldstrength of the magnetic lens approximately once per second. Themagnetic lens may be configured and used as described herein. Forexample, magnetic lens 100 may be configured to apply a magnetic fieldto a charged particle beam, and the charged particle beam may beconfigured to travel through the magnetic lens. Magnetic lens 100 mayalso be configured to operate as a magnetic condenser lens or a magneticobjective lens. Therefore, the magnetic lens may be configured to reduceaberrations in a charged particle beam, such as chromatic aberration andstigmation. The apparatus, however, may also be coupled to any magneticlens configured to generate a magnetic field when a current is appliedto the magnetic lens.

Apparatus 96 may be coupled to a magnetic lens and to a system, whichmay utilize the magnetic lens during use, such as a scanning electronmicroscope, a tunneling electron microscope, a focused ion beam device,or any other system which may be configured to inspect or fabricate aspecimen such as a semiconductor device using a charged particle beam.The semiconductor device may be fabricated, prior to inspection, using asemiconductor manufacturing process, such as lithography, etch, ionimplantation, deposition, chemical-mechanical polishing or plating.Additionally, the semiconductor device may be a portion of a device thatmay be formed on a semiconductor substrate. Alternatively, thesemiconductor device may be a working device which may be formed on asemiconductor substrate during semiconductor manufacturing. The magneticlens, however, may also be coupled to any system which utilizes acharged particle beam during operation such as an e-beam lithographysystem.

The system may include at least one magnetic lens. The magnetic lens maybe configured to apply a magnetic field to a charged particle beam. Thesystem may also include a charged particle beam source configured toproduce the charged particle beam. The system may be further configuredsuch that the charged particle beam may be configured to travel throughthe magnetic lens prior to interacting with the specimen. In addition,the system may include a stage configured to support at least a portionof the specimen. The stage may also be positioned along the path of thecharged particle beam such that the charged particle beam may interactwith the specimen. The system may also include any of the devices asdescribed and shown in FIG. 3 including, but not limited to,electrostatic or electromagnetic deflectors, beam-limiting apertures,Wien filters, magnetic condenser lenses, Schottky solid state barrierdetectors, an operating system, and an imaging device.

Magnetic sensor 98 may be configured to generate an output signalresponsive to a magnetic field strength of magnetic lens 100. Any signalthat may be responsive to a magnetic field strength or another conditionor property of the magnetic lens may be a mathematical representation ofthe magnetic field strength or another condition or property of themagnetic lens. For example, a signal may be linearly, proportionally,inversely, or logarithmically related to a condition or property of themagnetic lens. The magnetic sensor may be configured to operate as aHall-effect sensor and may provide an output signal (e.g., a voltage).The output signal may be linearly and proportionally related to themagnetic field strength within the magnetic lens. The output signal ofmagnetic sensor 98 may be an analog signal or a digital signal. Examplesof suitable Hall-effect sensors may be commercially available fromAllegro Microsystems, Inc. of Worcester, Mass. Other suitable magneticsensors, however, may also be used, which provide an output signalresponsive to the magnetic field strength of the magnetic lens. Themagnetic sensor may be disposed within a magnetic fringe field area ofthe magnetic lens, as shown in FIG. 5. Alternatively, the magneticsensor may be disposed within a cavity of the magnetic lens. The cavitymay be interposed between an inner pole piece of the magnetic lens andan outer pole piece of the magnetic lens. Additionally, the cavity maybe disposed within the inner pole piece of the magnetic lens.

The apparatus may also include a temperature sensor to measuretemperature variations of the magnetic lens. In an embodiment, apparatus96 may include temperature sensor 104 coupled to the magnetic lens andthe magnetic sensor. Temperature sensor 104 may be a matchedtemperature-dependent circuit element. Other suitable temperaturesensors, however, may also be included in the apparatus. Temperaturesensor 104 may be configured to monitor the temperature of the magneticlens and to generate an output signal, or a temperature signalresponsive to a temperature of the magnetic lens. Therefore, magneticsensor 98 may be further configured to receive the temperature signalfrom temperature sensor 104. In this manner, temperature sensor 104 maybe positioned proximal magnetic sensor 98 such that magnetic sensor 98and temperature sensor 104 may be exposed to an approximately equaltemperature of the magnetic lens 100. Furthermore, magnetic sensor 98may be further configured to generate an output signal responsive to themagnetic field strength of the magnetic lens in addition to thetemperature of the magnetic lens. As such, the output signal of magneticsensor 98 may be altered to include variations in the temperature of themagnetic lens.

Control circuit 102 coupled to the magnetic sensor may be configured toreceive the output signal from magnetic sensor 98. The control circuitmay also include a low-pass filter element (not shown) configured toreceive the output signal from the magnetic sensor. The low-pass filterelement may also be configured to reduce fluctuations in the outputsignal from magnetic sensor 98. Therefore, the low-pass filter elementmay prevent fluctuations in the output signal from the magnetic sensorfrom being transferred into output signals generated by the controlcircuit. The control signal may be configured to generate an outputsignal in response to the output signal from the magnetic sensor. Thesignals generated by the control circuit may be used to control themagnetic lens. As such, the low-pass filter element may preventfluctuations in the current supplied to the magnetic lens, which mayadversely affect the performance of the magnetic lens.

Control circuit 102 may also be configured to receive input signal 106,which may be responsive to a predetermined magnetic field strength. Anoperating system (not shown) may be coupled to the apparatus and may beconfigured to generate input signal 106. The operating system may bemanually-controlled or computer-controlled. Input signal 106 may includea voltage, which may have a linear and proportional relationship to thepredetermined magnetic field strength of the magnetic lens. Thepredetermined magnetic field strength may be a variable magnetic fieldstrength or a constant magnetic field strength. Therefore, thepredetermined magnetic field strength may vary in response to a desiredperformance of the magnetic lens, which may also vary over time. Inaddition, the predetermined magnetic field strength may be constant inresponse to a desired performance of the magnetic lens, which may besustained over a period of time. In this manner, the control circuit maybe configured to alter the magnetic field strength of the magnetic lensor to maintain the magnetic field strength of the magnetic lens.

The predetermined magnetic field strength may be determined by anoperator. For example, the manually-controlled operating system may beconfigured to provide information, which may describe the performance ofthe magnetic lens, to an operator. The operator may determine thedesired performance or function of the magnetic lens by analyzing theinformation about the performance of the magnetic lens. Themanually-controlled operating system may be further configured toreceive an input signal from the operator representative of a desiredperformance or function of the magnetic lens. The input from theoperator may also be representative of a predetermined magnetic fieldstrength, which may enable the desired performance of the magnetic lens.The manually-controlled operating system may, therefore, be configuredto then convert the input from the operator to an input signal, whichmay be received by the control circuit.

Alternatively, the predetermined magnetic field strength may bedetermined by a computer-controlled operating system. Thecomputer-controlled operating system may include a computer such as apersonal computer or a mainframe computer. For example, thecomputer-controlled operating system may be configured to receiveinformation which may be descriptive of the performance of the magneticlens. The computer-controlled operating system may be further configuredto analyze the information and to generate an input signalrepresentative of a desired performance or function of the magneticlens. The input from the computer-controlled operating system may,therefore, be representative of the differences between the desiredperformance of the magnetic lens and the current performance of themagnetic lens. As such, the input signal from the computer-controlledoperating system may also be representative of a predetermined magneticfield strength, which may enable the desired performance or the desiredfunction of the magnetic lens.

Control circuit 96 may also be configured to generate a control signalresponsive to the output signal from magnetic sensor 98 and input signal106. In addition, control circuit 96 may be configured to generate acontrol signal responsive to the output signal and the input signal. Thecontrol circuit may also include operational amplifier 108 configured toreceive the output signal from magnetic sensor 98 and input signal 106.The operational amplifier may be further configured to generate acomparison signal, which may be responsive to differences between theoutput signal from the magnetic sensor and the input signal from theoperating system. For example, the operational amplifier may perform anynumber of comparisons between the output signal and the input signalincluding, but not limited to, subtraction, multiplication, division,and algorithms. Operational amplifier 108 may also be configured togenerate a control signal, which may be a function of the comparisonsignal. For example, the operational amplifier may be configured tocompare the output signal from the magnetic sensor and the input signaland to apply a gain to a difference between the two signals.Alternatively, the control circuit may include any circuit element or aplurality of circuit elements, which may be configured to perform theoperations described herein.

Control circuit 96 may also be configured to drive magnetic lens 100 byapplying a current to at least one coil of the magnetic lens. Thecurrent may be responsive to the control signal, which may be generatedby the control circuit. The applied current may be effective to generatea magnetic field strength within the magnetic lens that is closer to thepredetermined magnetic field strength than the measured magnetic fieldstrength. Alternatively, the applied current may be effective togenerate a magnetic field strength within the magnetic lens that issubstantially equal to the predetermined magnetic field strength.Control circuit 96 may also include an electronic current drive system(not shown), which may be configured to receive the control signal fromthe control circuit and to drive the magnetic lens by applying a currentto at least one coil of the magnetic lens. As such, the electroniccurrent drive system may receive the control signal from operationalamplifier 108, which may be included in control circuit 96. In addition,the electronic current drive system may be configured to perform anoperation on the control signal. For example, the electronic currentdrive system may be configured to convert the control signal from ananalog signal to a digital signal. The electronic current drive systemmay be further configured to alter and control a power source inresponse to the control signal. The power source may be configured tosupply a current to at least one coil of the magnetic lens. Therefore,the current, which may be supplied to the magnetic lens, may be alteredand/or controlled by the control circuit.

FIG. 6 a illustrates an embodiment of a method for controlling amagnetic field strength of a magnetic lens. The method may be used tocontinuously or to intermittently control the magnetic field strength ofa magnetic lens. Prior to performing the method, a magnetic field may begenerated within magnetic lens 110 by applying a current to at least onecoil winding such as primary coil winding 134 of the magnetic lens. Thecurrent may be a current applied to magnetic lens 110 prior to beginningthe method such as a current appropriate for starting up a magnetic lensor a system in which the magnetic lens is used. The current may also bea current applied to magnetic lens 110 prior to beginning the methodsuch as a current appropriate for a prior use of the magnetic lens. Asshown in step 112, the method may include generating an output signalresponsive to the magnetic field strength of magnetic lens 110. Theoutput signal may be generated by using magnetic sensor 114. The outputsignal generated by the magnetic sensor may be a voltage, which may havea linear relationship to the magnetic field strength of the magneticlens.

Magnetic sensor 114 may be disposed within a magnetic field of magneticlens 110. For example, magnetic sensor 114 may be disposed within amagnetic fringe field area of magnetic lens 110, as shown in FIG. 6 a.Alternatively, magnetic sensor 114 may be disposed within a cavity ofthe magnetic lens. The cavity may be defined as a space between an outerpole piece of the magnetic lens and an inner pole piece of the magneticlens. As shown in FIG. 6 b, magnetic sensor 114 is disposed between anouter pole piece and an inner pole piece of magnetic lens 110. Inaddition, magnetic sensor 114 may be disposed within an inner pole pieceof the magnetic lens. As shown in FIG. 6 c, magnetic sensor 114 isdisposed within an inner pole piece of magnetic lens 110. In thismanner, magnetic sensors may be located at different positions internaland external to the magnetic lens.

In an embodiment, the method may include generating an output signalwhich may be responsive to a temperature of the magnetic lens, as shownin step 116. The temperature signal may be generating by usingtemperature sensor 118. The generated temperature signal may beresponsive to a temperature of magnetic lens 110. The temperature sensormay be coupled to the magnetic lens and the magnetic sensor.Furthermore, the temperature sensor may send the generated temperaturesignal to the magnetic sensor. As a result, the magnetic sensor mayalter the output signal of the magnetic sensor in response to thetemperature signal. For example, the temperature signal may be used toalter the output signal of the magnetic sensor such that the magneticsensor may be sensitive to fluctuations in the temperature of themagnetic lens. As such, temperature sensor 118 may be positionedproximal magnetic sensor 114. In this manner, the temperature signalgenerated by temperature sensor 118 may be responsive to a temperatureof magnetic lens 110 proximal magnetic sensor 114 and temperature sensor118. For example, as shown in FIG. 6 a, temperature sensor 118 andmagnetic sensor 114 may be placed within a magnetic fringe field area ofmagnetic lens 110. Alternatively, as shown in FIG. 6 b, temperaturesensor 118 and magnetic sensor 114 may be disposed within a cavity ofthe magnetic lens. In addition, as shown in FIG. 6 c, temperature sensor118 and magnetic sensor 114 may be disposed within an inner pole pieceof magnetic lens 110.

In an additional embodiment, the method may include reducingfluctuations in the output signal, as shown in step 120. The outputsignal may be sent to a low-pass circuit element, which may be coupledto a control circuit. Reducing fluctuations in the output signal mayprovide improved control on the magnetic lens by eliminating erraticchanges in the current being applied to at least one coil of themagnetic lens. In addition, the method may include sending the outputsignal to a control circuit, as shown in step 122. The control circuitmay be coupled to the magnetic sensor and the magnetic lens.

In an embodiment, the method may include sending an input signal to thecontrol circuit, as shown in step 124. The input signal may beresponsive to a function of a predetermined magnetic field strength. Forexample, the predetermined magnetic field strength may be representativeof a desired function or operating characteristic of the magnetic lens.Alternatively, the predetermined magnetic field strength may also be asubstantially constant magnetic field strength. In this manner, themethod may be used to eliminate variations in the magnetic fieldstrength of the magnetic lens over time. The input signal may be avoltage, which may have a linear relationship to the predeterminedmagnetic field strength of the magnetic lens. The input signal may begenerated by using an operating system, which may be coupled to thecontrol circuit. The operating system may be manually-controlled orcomputer-controlled. As such, the input signal which may be determinedby an operator or by a controller computer.

As shown in step 126, the method for controlling the magnetic fieldstrength of a magnetic lens may include generating a control signal. Thecontrol signal may be responsive to a function of the output signal andthe input signal. In an embodiment, an operational amplifier may becoupled to the control circuit. Therefore, generating the control signalmay include generating a comparison signal by using the operationalamplifier. The operational amplifier may be configured to generate thecontrol signal by comparing the output signal from the magnetic sensorand the input signal. As such, the operational amplifier may generate acomparison signal, which may be responsive to differences between theoutput signal and the input signal. The operational amplifier may alsobe configured to perform a function on the generated comparison signal.For example, the operational amplifier may generate the control signalby applying a gain to the comparison signal.

In an embodiment, the method may also include sending the control signalto an electronic current drive system, as shown in step 128. Theelectronic current drive system may be coupled to the control circuit.The electronic current drive system may be used to control the current,which may be applied to at least one coil of the magnetic lens. In anembodiment, as shown in step 130, the method may include applying acurrent to at least one coil of the magnetic lens. The current may beresponsive to the control signal which may be generated by the controlcircuit. Therefore, as shown in step 132, the predetermined magneticfield strength may be generated within the magnetic lens. For example,the current may be effective to generate a magnetic field strengthwithin the magnetic lens that is closer to the predetermined magneticfield strength than a first or measured magnetic field strength. Inaddition, the current may also be effective to generate a magnetic fieldstrength within the magnetic lens that is substantially equal to thepredetermined magnetic field strength.

In an embodiment, the method for controlling a magnetic field strengthof a magnetic lens may also include directing a charged particle beamthrough the magnetic lens. The charged particle beam may be an electronbeam or an ion beam. In this manner, the magnetic lens may be used toapply a magnetic field to the charged particle beam. As such, themagnetic lens may be coupled to a device, which may use a magnetic lensto alter the path of a charged particle beam. Examples of such devicesmay include, but are not limited to, scanning electron microscopes,tunneling electron microscopes, e-beam lithography devices or focusedion beam devices. For example, the magnetic lens may be configured tooperate as a magnetic lens incorporated into a scanning electronmicroscope.

Inspecting a specimen such as a semiconductor device, which may beformed on a semiconductor substrate, subsequent to intermediate andfinal processing steps has become an integral part of successfullyproducing working semiconductor devices. The semiconductor device may befabricated prior to inspection by using a semiconductor manufacturingprocess. The semiconductor manufacturing process may include a number ofprocessing steps as described herein. The semiconductor device may alsoinclude a semiconductor topography, which may be only a portion of aworking semiconductor device. For example, a semiconductor device may beinspected subsequent to each of the many processing steps required tofabricate a working semiconductor device.

In an embodiment, the method for inspecting a specimen may includegenerating a magnetic field within the magnetic lens by applying acurrent to at least one coil of the magnetic lens. In addition, themethod may also include applying the generated magnetic field to acharged particle beam by directing the charged particle beam through themagnetic lens. The charged particle beam may be an electron beam or anion beam. The method may also include generating the charged particlebeam by using a charged particle beam source. Prior to directing thecharged particle beam through the magnetic lens, at least a portion of aspecimen may be positioned on a stage configured to support thespecimen. The stage may be located along a path of the charged particlebeam. As such, the charged particle beam may impinge upon the specimenafter passing through the magnetic lens. The charged particle beamexiting the magnetic lens may be substantially free of aberrations.Therefore, the magnetic lens may be coupled to a scanning electronmicroscope or other suitable device, which may apply a magnetic field toa charged particle beam to alter a path of the charged particle beam, orto reduce aberrations that may be present in the charged particle beam.

Inspecting a semiconductor device may also include monitoring andaltering a magnetic field strength of the magnetic lens. Monitoring andaltering the magnetic field strength of the magnetic lens may includegenerating an output signal, which may be responsive to a magnetic fieldstrength of the magnetic lens by using a magnetic sensor. The magneticsensor may be disposed within a magnetic field of the magnetic lens. Themethod may also include sending the output signal to a control circuit,which may be coupled to the magnetic sensor and the magnetic lens. Inaddition, the method may include sending an input signal to the controlcircuit. The input signal may be responsive to a function of apredetermined magnetic field strength. The method may also include usingthe control circuit to generate a control signal which may be responsiveto a function of the output signal and the input signal. Furthermore,the method may include generating the predetermined magnetic fieldstrength within the magnetic lens by applying a current to at least onecoil of the magnetic lens. The current may be responsive to the controlsignal. The method may also include additional steps as describedherein.

The apparatus and methods for using the apparatus described above mayprovide accurate control of the magnetic field within a current-drivenmagnetic lens. The magnetic lens may be coupled to any device which usesa charged particle beam to perform a function. Using the apparatus andmethods to control a magnetic field of a magnetic lens may eliminateadverse effects of hysteresis on the performance of the magnetic lens.In addition, the apparatus and methods may also reduce the effects oftemperature dependent material properties, drift in current driveelectronics, low frequency noise, eddy currents, undesired superimposedfields on the magnetic field generated by a magnetic lens. Furthermore,the apparatus and methods described above may also be used to eliminatedrift in the magnetic field strength over time from other causes. In theapplication to charged particle beam devices, this functionality may beuseful for reproducibly tuning magnetic components as magnetic lenses,Wien filters, or deflection coils. Specifically, the magnetic fieldsensor may permit easier manual and automated operation of scanningelectron microscopes and other electron beam devices. The magneticfeedback concept may greatly increase tool stability and tool to toolconsistency. Specifically, optimized values of magnetic field fordeflection coils, Wien filters and magnetic lenses may be readilyreproduced. Additionally, using a magnetic field sensor may eliminatethe need to couple a current sensing resistor to the control circuit.Therefore, the magnetic lens may be driven with the same current but byapplying a lower voltage to the magnetic lens.

EXAMPLE Effect of Magnetic Field Feedback Control on a Magnetic Lens

FIG. 7 illustrates a plot of the hysteresis of a magnetic lens using acurrent feedback control apparatus. The magnetic field strength wasestimated by measuring the current in the lens coil. The current in thelens coil was varied to alter the magnetic field strength of themagnetic lens. Variations in the current in the lens coil were highestat the beginning of testing and were decreased over time. The firstcurrent, which was applied to the lens coil, has been designated as thepoint on the graph which has been labeled “start.” The last current,which was applied to the lens coil, has been designated as the point onthe graph which has been labeled “end.” The measured magnetic field hasbeen plotted versus the desired magnetic field. The range of themeasured magnetic field and the desired magnetic field range fromapproximately −10 gauss to approximately +10 gauss. As shown in FIG. 7,the measured magnetic field has a non-linear relationship to the desiredmagnetic field. The non-linear relationship may be caused by the factthat instantaneous magnetic field strength is determined by the presentcurrent and the history of all other currents in the coil and iscommonly referred to as hysteresis.

FIG. 8 illustrates a plot of the hysteresis of a magnetic lens using amagnetic field feedback control apparatus. The magnetic field strengthwas estimated by measuring the magnetic field of the magnetic lens usingan Allegro Hall Sensor (Allegro Microsystems, Inc., Worcester, Mass.).The current in the lens coil was varied to alter the magnetic fieldstrength of the magnetic lens. Variations in the current in the lenscoil were highest at the beginning of testing and were decreased overtime. The measured magnetic field has been plotted versus the desiredmagnetic field. The range of the measured magnetic field and the desiredmagnetic field is from approximately −10 gauss to approximately +10gauss. As shown in FIG. 8, by using a magnetic field feedback controlapparatus, a linear relationship between the measured magnetic field andthe desired magnetic field was established. The linear relationshipbetween the measured magnetic field and the desired magnetic fieldindicates that the effects of hysteresis in the magnetic lens wereeffectively minimized. Therefore, by implementing a control method,which includes using a magnetic field feedback control apparatus asdescribed herein, hysteresis in the magnetic field strength of themagnetic lens may be substantially eliminated.

It will be appreciated to those skilled in the art having the benefit ofthis disclosure that this invention is believed to provide a magneticlens having at least one pole piece which has at least two sectors andan apparatus configured to control the magnetic field strength of amagnetic lens. Further modifications and alternative embodiments ofvarious aspects of the invention will be apparent to those skilled inthe art in view of this description. For example, the structure of amagnetic lens may also be applied to electrostatic devices, such asdeflectors, or devices, which generate both magnetic and electrostaticpotentials, such as Wien filters. In addition, the apparatus, which maymonitor and control a magnetic lens using magnetic field feedbackcontrol, may be integrated into any device which generates a magneticfield during operation. It is intended that the following claims beinterpreted to embrace all such modifications and changes and,accordingly, the specification and drawings are to be regarded in anillustrative rather than a restrictive sense.

1. An apparatus configured to control a magnetic field strength of amagnetic lens during use, comprising: a magnetic sensor disposed withina magnetic field generated by the magnetic lens, wherein the magneticsensor is further disposed within the magnetic lens, wherein themagnetic sensor is configured to generate an output signal during use,wherein the output signal is responsive to a first magnetic fieldstrength of the magnetic field generated by the magnetic lens, andwherein the magnetic field will be applied to a charged particle beamtraveling through the magnetic lens; and a control circuit coupled tothe magnetic sensor and the magnetic lens, where in the control circuitis configured: to receive the output signal from tho magnetic sensorduring use; to receive an input signal responsive to a predeterminedmagnetic field strength during use; to generate a control signalresponsive to the output signal and the input signal during use; and toapply a current to the magnetic lens, wherein the current is responsiveto the control signal.
 2. The apparatus of claim 1, wherein the magneticlens is coupled to a scanning electron microscope.
 3. The apparatus ofclaim 1, wherein the input signal comprises a voltage having a linearrelationship to the predetermined magnetic field strength of themagnetic lens.
 4. The apparatus of claim 1, wherein the output signalcomprises a voltage having a linear relationship to the first magneticfield strength of the magnetic lens.
 5. The apparatus of claim 1,wherein the control signal is further responsive to a function of theoutput signal and the input signal.
 6. The apparatus of claim 1, whereinthe control circuit is further configured to apply a current to at leastone coil of the magnetic lens.
 7. The apparatus of claim 1, wherein thecurrent is effective to generate a second magnetic field strength withinthe magnetic lens, and wherein the second magnetic field strength iscloser to he predetermined magnetic field strength than the firstmagnetic field strength.
 8. The apparatus of claim 1, wherein thecurrent is effective to generate a second magnetic field strength withinthe magnetic lens, and wherein the second magnetic field strength issubstantially the same as the predetermined magnetic field strength. 9.The apparatus of claim 1, wherein the apparatus is further configured tocontinuously control the magnetic field strength of the magnetic lensduring use.
 10. The apparatus of claim 1, wherein the apparatus isfurther configured to intermittently control the magnetic field strengthof the magnetic lens during use.
 11. The apparatus of claim 1, whereinthe magnetic sensor is further disposed within a cavity in the magneticlens, and wherein the cavity is disposed between an outer pole piece ofthe magnetic lens and an inner pole piece of the magnetic lens.
 12. Theapparatus of claim 1, wherein the magnetic sensor is further disposedwithin an inner pole piece of the magnetic lens.
 13. The apparatus ofclaim 1, further comprising a temperature sensor coupled to the magneticlens, wherein the temperature sensor is configured to generate atemperature signal during use, and wherein the temperature signal isresponsive to a temperature of the magnetic lens.
 14. The apparatus ofclaim 13, wherein the temperature sensor is further coupled to themagnetic sensor, wherein the magnetic sensor is further configured toreceive the temperature signal during use, and wherein the output signalis further responsive to the temperature of the magnetic lens.
 15. Theapparatus of claim 1, wherein the control circuit comprises a low-passcircuit element configured to receive the output signal during use andto reduce fluctuations in the output signal during use.
 16. Theapparatus of claim 1, wherein the control circuit comprises anoperational amplifier configured to generate a comparison signal duringuse, wherein the comparison signal is responsive to a comparison of theoutput signal and the input signal, and wherein the control signal isfurther responsive to a function of the comparison signal.
 17. Theapparatus of claim 1, wherein the control circuit comprises anelectronic current drive system configured to receive the control signalduring use and to apply the current to the magnetic lens during use. 18.A method for controlling a magnetic field strength of a magnetic lens,comprising: generating an output signal in response to a first magneticfield strength of a magnetic field generated by the magnetic lens,wherein the magnetic field will be applied to a charged particle beamtraveling through the magnetic lens, and wherein said generating isperformed by a magnetic sensor disposed within the magnetic lens;generating an input signal in response to a predetermined magnetic fieldstrength; generating an input signal in response to the output signaland the input signal; and applying a current to the magnetic lens,wherein the current is responsive to the control signal.
 19. A systemconfigured to inspect a specimen during use, comprising: at least onemagnetic lens configured to apply a magnetic field to a charged particlebeam during use, wherein the magnetic lens is positioned along a path ofthe charged particle beam; and an apparatus configured to control amagnetic field strength of the magnetic field generated by the magneticlens during use, wherein the apparatus is coupled to the magnetic lensand the system, the apparatus comprising: a magnetic sensor disposedwithin the magnetic field generated by the magnetic lens, wherein themagnetic field will be applied to the charged particle beam travelingthrough the magnetic lens, wherein the magnetic sensor is furtherdisposed within the magnetic lens, wherein the magnetic sensor isconfigured to generate an output signal during use, and wherein theoutput signal is responsive to a first magnetic field strength of themagnetic field generated by the magnetic lens; and a control circuitcoupled to the magnetic sensor and the magnetic lens, wherein thecontrol circuit is configured: to receive the output signal from themagnetic sensor during use; to receive an input signal responsive to apredetermined magnetic field strength during use; to generate a controlsignal responsive to the output signal and the input signal during use;and to apply a current to the magnetic lens, wherein the current isresponsive to the control signal.
 20. A method for inspecting aspecimen, comprising: generating a magnetic field by a magnetic lens andapplying the magnetic field to a charged particle beam, wherein applyingthe magnetic field to the charged particle beam comprises directing thecharged particle beam through the magnetic lens; and controlling amagnetic field strength of the magnetic field of the magnetic lens,comprising: generating an output signal in response to a first magneticfield strength of the magnetic field generated by the magnetic lens,wherein said generating is performed by a magnetic sensor disposedwithin the magnetic lens; generating an input signal in response to apredetermined magnetic field strength; generating a control signal inresponse to the output signal and the input signal; and applying acurrent to the magnetic lens, wherein the current is responsive to thecontrol signal.